The present invention relates generally to computer networks, and more specifically to a system and method for reducing data transfer latency and network-induced jitter in computer networks.
Data processing devices such as personal computers and workstations are increasingly called upon to retrieve object data from mass storage devices over computer networks. Because the speed at which such mass storage devices can transfer data objects to data processing devices over networks is limited by various factors including the reliability of the network, the traffic congestion on the network, and the load on the mass storage device, data processing devices typically employ local data storage devices such as cache memory for storing copies of frequently requested data objects. By retrieving stored copies of popular data objects from local cache memory, data processing devices can reduce the level of traffic congestion on the network, the amount of load on mass storage devices, and the latency of the data transfer.
Conventional localized cache types include block caches, file caches, and disk caches. In a typical block cache, data is organized in a plurality of data blocks, each of which has a size larger than that of the object data stored therein. When a data processing device is called upon to retrieve a particular data object, the device attempts to find a copy of the requested data object in one of the data blocks stored in the block cache. In the event the requested data object cannot be found in the local block cache (commonly known as a “cache miss”), the block cache retrieves a block of data including the requested data object from a mass storage device over the network, stores a copy of the retrieved data block in the block cache, and delivers the requested data object to the data processing device. In the event the data object is found in the local block cache (commonly known as a “cache hit”), the block cache delivers the requested data object directly to the data processing device without accessing any data from the mass storage device. It is noted that when the local block cache is full and there is a cache miss, the block cache typically retrieves a data block containing the requested data object from the mass storage device, stores a copy of the data block in the block cache by overwriting data within the block cache, and delivers the requested data block to the data processing device. For example, the block cache may overwrite data currently stored in the block cache based on a least-used algorithm.
In a typical file cache, data is organized in a plurality of files, and when the data processing device is called upon to retrieve a particular data object, it attempts to find the requested data object in one of the files stored in the file cache. In the event of a cache miss, the file cache employs an appropriate filename to retrieve the entire file including the requested data object from a mass storage device over the network, stores a copy of the retrieved file in the file cache, and delivers the requested data object to the data processing device. In the event of a cache hit, the file cache delivers the requested data object immediately to the data processing device without accessing the mass storage device.
In a typical disk cache, data is also organized in a plurality of data blocks. When called upon to retrieve a particular data object, the data processing device attempts to find the requested data object in one of the data blocks stored in the disk cache. In the event of a cache miss, the disk cache retrieves a block of data including the requested data object from a mass storage device such as a disk, stores a copy of the retrieved data block in the disk cache, and delivers the requested data object to the data processing device. In disk caching, not only is the data block containing the requested data object stored in the disk cache, but data blocks in at least one adjacent sector of the disk are also stored in the disk cache (commonly known as “pre-fetching”). In the event of a cache hit, the disk cache delivers the requested data object in a stored or pre-fetched data block immediately to the data processing device without accessing the disk.
One drawback of the conventional localized cache types is that they generally cannot handle retrieval, storage, and delivery of large object data types such as multimedia objects containing audio, video, text, and/or graphics. This is because the size of a single multimedia object can be larger than the total storage capacity of the block, file, or disk cache local to the data processing device.
In an attempt to overcome the limited storage capacity of conventional localized caches, large shared disk resources have been provided on networks at strategic locations for caching large data objects. Further, data streaming techniques have been employed to transfer large data objects over networks from mass storage devices to data processing devices via local caches and/or large shared disk resources. Although these techniques may be successfully used to transfer non-real-time multimedia objects containing text and/or graphics, such techniques often have problems when transferring real-time multimedia objects containing audio and/or video, which generally require a steady rate of data transfer to avoid network-induced jitter in the multimedia data stream. Such jitter can cause intolerable delay and/or distortion upon playback of the audio or video signal.
One way of reducing network-induced jitter in audio data streams is to employ a known data streaming technique that uses a jitter buffer local to a data processing device for storing a sufficient amount of audio data to allow constant playback of the audio signal during worst-case variations in the data transfer rate. For example, if a worst-case variation in the data transfer rate during delivery of an audio data object is 30 msecs, then the above-described data streaming technique may store an amount of audio data corresponding to the first 30 msecs of audio playback in the jitter buffer before playing-back the audio signal. A variable delay having a maximum value of 30 msecs is then applied to the audio data as it passes through the jitter buffer to counteract the variable delay applied to the audio data by the network.
However, such jitter buffers also have drawbacks in that they typically do not store object data with a level of persistence that allows quick delivery of the data upon subsequent data requests. Further, jitter buffers like that employed in the above-described data streaming technique generally increase data transfer latency by the interval of time required to store the initial amount of audio data in the jitter buffer.
It would therefore be desirable to have a system and method for reducing data transfer latency and network-induced jitter in computer networks. Such a system would be capable of handling the transfer of large object data types, e.g., text and graphics for non-real-time applications and audio and video for real-time applications, without requiring a large amount of local data storage.